The invention concerns a process to determine aerosol particles in gaseous samples, specifically in the exhaust of diesel motors, including the at least indirect deposit of aerosol particles on a piezoelectric oscillating crystal and the determination of the oscillation parameters of the piezoelectrically stimulated oscillation of the crystal element.
Emissions of diesel motors are classified primarily by the particle concentration and the particle number, where the measurement of the particle concentration places increasingly more stringent demands upon the technology of measurement. It is presently not possible to carry out a measurement at low emission levels and/or a measurement with high temporal resolution.
The particle concentration is normally measured by the deposition of particles in filters or by deposits on substrata. Deposits on substrata involve impact or electrostatic deposits in most measurement processes. The amount of particles deposited is then measured as the difference of the weight of the filters or substrata before and after the exposure, from which the concentration can then be calculated.
These measurement processes yield the accumulated particle weight within a measurement cycle. The disadvantages of both methods are the high degree of imprecision of the weight measurement, especially at low particle weights, and the lack of temporal resolution within a measurement cycle.
Use of the crystal micro scale is a process that does not exhibit the disadvantages specified above; it provides a signal with high temporal resolution and is especially well suited for the determination of small weights. This process uses the weight sensitivity of a piezoelectric crystal, which is equipped with electrodes and which may be charged into mechanical oscillations, onto which the particles are deposited. The deposited particle layer causes the resonance frequency of the crystal element to decrease, where the amount of decrease provides a measurement of the deposited particle weight.
This process is described in U.S. Pat. No. 3,561,253 and in a novel arrangement in EP 0 779 510 A2. According to U.S. Pat. No. 3,561,253, the deposition of particles on the quartz crystal, which is equipped with electrodes, involves impact or electrostatic deposits. The measurement of the change in resonance frequency of the system, consisting of the quartz crystal and the oscillator, is based on a frequency counter. EP 0 779 510 A2 describes an apparatus consisting essentially of a deposit component, an oscillating element with means to determine the frequency and a heating element; this apparatus allows for a subsequent thermal gravimetric analysis. As EP 0 779 510 A2 points out, such apparatus produces changes in the resonance frequency of merely a few hundred Hertz with injection of diesel particles.
The capabilities of such a system are shown in the determination of layer thickness in thin film technology. Such sensors yield modifications of the resonance frequency of up to 1 MHz, the layer thickness can be determined to just a few nm, and the behavior of the sensor due to the deposited layer can be described theoretically with great precision, assuming that the deposited layers have the properties of solids and that they can be produced in a reproducible manner.
However, this is normally not the case for diesel particles, because normally the formation involves flocculation and accumulation in a layer structure that is not very dense. It can be shown that the oscillation of the crystal will not compress the deposited particle layer, as would have been expected.
It is an object of the present invention to present an improved process to measure aerosol particles, which guarantees the required reproducibility and thus a high quality of measurements that is reliable and independent of the type of aerosol particles.
This objective is achieved by stimulating the piezoelectric crystal to oscillation during less than half the period of deposits of the aerosol particles. This process significantly improves the deposit of particles on the crystal during rest periods of the crystal and leads to the formation of a homogeneous particle layer on the surface, which enlarges the measurement range and significantly improves the reproducibility of the measurement.
According to an additional characteristic of the invention, it is possible to design the system such that a portion of the sample is fed into a second apparatus that takes continuous measurements, where at least one of these measurement values is calibrated gravimetrically by use of at least one measurement value that was determined from the oscillation parameters of the piezoelectrically stimulated oscillation of the crystal.
Many types of continuous processes have the disadvantage that it is generally not possible to derive a gravimetric signal from the measurement values, because a change of the system to be measured causes such changes in the measurement values that it is not possible to derive a specification of a generally valid calibration factor relating the measurement value and the gravimetric signal to be derived from it. However, for short time intervals and for known weights of aerosol particles, it is possible to derive a calibration factor applicable to that time interval. It is advantageous then to determine a calibration factor for the continuous measurement process during specific time intervals from the weight deposited on the piezoelectric crystal.
A particularly advantageous embodiment provides for the continuous measurement by the at least indirect deposit of aerosol particles on a second piezoelectric oscillating crystal and the determination of the oscillation parameters of the piezoelectrically stimulated oscillation of the second crystal element.
However, the objective stated above is also achieved by an additional characteristic of the invention, namely that at least a portion of the sample is fed to a second apparatus that takes continuous measurements, where at least one of these measurement values is calibrated gravimetrically by at least one of the measurement values determined from the oscillation parameter of the piezoelectrically stimulated oscillation of the crystal element. This calibration guarantees the reproducibility of the measurements of the continuous process independent of potential modifications of the measurement value not caused by the system to be measured.
A further characteristic of the invention enables a variation that is particularly efficient at saving time, where the sample is divided and is fed to both measurement processes simultaneously. This variation is also suited for the combination of measurement processes, which do not modify the sample, with processes, which modify the sample in definable parameters, such as by deposits on a piezoelectric crystal or by deposit in a filter.
If only small volumes or amounts of the sample are available, an alternative embodiment of the present invention will often be more useful, where the sample is fed sequentially to both measurement processes. However, it is significant here that the first process in the sequence does not modify the sample or modifies it only in those properties that are not to be measured by the subsequent process.
A first embodiment of the invention as a continuous measurement process relies on the determination of the quantity of light dispersed by the aerosol particles.
An alternative embodiment of the invention in turn uses a process where the continuous measurement process relies on the determination of the quantity of light absorbed by the aerosol particles in the gaseous sample.
It is advantageous for both of the just specified embodiments of the invention that the sample is first fed to the continuous measurement process and subsequently to the process with at least indirect deposit of aerosol particles on a piezoelectric oscillating crystal, because the continuous measurement processes in question will not modify the properties to be measured by the piezoelectric process.